1. Field of the Invention
The present invention relates to a frequency synthesizer having a multi-band voltage controlled oscillator, and more particularly, to a frequency synthesizer including a multi-band voltage controlled oscillator having different voltage controlled oscillating cores according to a frequency band, which can automatically determine a frequency band where each of the voltage controlled oscillating cores operates.
2. Description of the Related Art
In general, a frequency synthesizer is a circuit that generates an output signal of a specific frequency having a predetermined phase corresponding to a reference frequency by using a phase locked loop (PLL). The frequency synthesizer with the phase locked loop may include a phase frequency detector (PFD), a charge pump (CP), a loop filter, a voltage controlled oscillator (VCO), a prescaler, and a divider. The phase frequency detector compares a pre-set reference frequency with a frequency obtained by dividing an output frequency of the frequency synthesizer output from the prescaler in a predetermined ratio in the divider, and then outputs an error signal corresponding to a difference from the comparison. The charge pump generates a control voltage of the voltage controlled oscillator depending on a size of the error signal, and the loop filter removes a high frequency component from the control voltage, and then the control voltage is input to the voltage controlled oscillator as a control signal of the voltage controlled oscillator. The voltage controlled oscillator generates a frequency corresponding to the control voltage and outputs the frequency, and the generated frequency is divided into a final desired frequency by the prescaler.
Since the voltage controlled oscillator of the frequency synthesizer includes one voltage controlled oscillating core, a range of frequencies that can be generated is limited. Particularly, as processes of manufacturing a voltage controlled oscillator are getting smaller, a range of frequencies of the voltage controlled oscillator that can be generated is being gradually limited. Thus, a voltage controlled oscillator applied to fields requiring a wide band of frequencies includes a plurality of voltage controlled oscillating cores that can generate different frequency bands from each other. Also, each of the voltage controlled oscillating cores are manufactured to divide a frequency band that each of the voltage controlled oscillating cores can generates into a plurality of bands and to operate according to each of the divided bands.
Meanwhile, the control voltage provided to the voltage controlled oscillator is limited within a predetermined range to obtain low noise characteristics. When a range of operation frequencies of the voltage controlled oscillator is changed by variations in a temperature or a process, it may be out of a range of frequencies generated by the control voltage within a predetermined range in an operation band of the voltage controlled oscillating core. In this case, the voltage controlled oscillator must change the operation band of the voltage controlled oscillating core generating an output frequency into another band of an appropriate frequency generated by the input control voltage, or change the operating voltage controlled oscillating core itself. Also, in a process of changing the operation band of the voltage controlled oscillating core, it is required to provide a bias current that optimizes noise characteristics of an output frequency of the relevant operation band.
Similarly, in the case of the prescaler, operation bands are divided according to an input frequency band. That is, the prescaler must set an appropriate operation band according to an input frequency band.
Thus, it is required in the art to develop a technology that can appropriately and automatically change an operation band of the operating voltage controlled oscillating core and provide an appropriate bias current for the changed band such that the voltage controlled oscillator outputting frequencies of multi-bands generates a frequency having excellent noise characteristics even when variations in a temperature and a process occur. It is also required in the art to develop a technology that can appropriately and automatically determine an operation band of the prescaler according to a frequency band generated by the voltage controlled oscillator.